Progressor and regressor fibrosarcoma tumors were observed in quails inoculated with virus-producing M-MSV transformed quail embryo cells. Antibodies directed against M-MSV and recombinant B-MuX protein products were detected. Two strains of M-MSV proviral DNA were cloned in lambda vectors. Host flanking sequences were different for each provirus which did contain terminal redundant sequences. Cloned MSV's were biologically active in transfection assays. FeSv specific cDNA probes were used to test FOCMA positive and negative cat lymphosarcomas. No FeSV src RNA was found in these tumors. Recombinant MuLV's were found in all leukemogenic MuLV stocks tested. These generally exhibited genomic masking by ecotropic gp70. Oncornavirus inactivating factor (OIF) was found to be lipid in nature. Seroimmunoprophylaxis experiments with hyperimmune gamma globulin directed against MuLV gp70 in AKR mice were successful. New treatment parameters were more effective and were related to reduction of several MuLV types.